1. Field of the Invention
The present invention relates to a method for preparing rare earth (hereinafter referred to as RE) 123-type oxide superconductors, including Y 123 oxide superconductors, which exhibit a high critical temperature of 90 K or above and have excellent superconductive properties in the atmosphere.
2. Description of the Related Art
Since oxide superconductors exhibiting a critical temperature higher than the temperature of liquid nitrogen (77.3 K) were recently found, superconductor applying technology has been receiving wider appreciation, thereby accelerating the competitive development of materials which exhibit stable superconductivity at higher temperatures.
Especially, in the case of a Y-Ba-Cu-O oxide superconductor as a 123-type oxide in which the mole ratio of Y:Ba:Cu in the material is 1:2:3, a higher critical current density (Jc) has been successfully achieved by improving a process for preparing such a superconductor, while the material has been used as current lead wires as a matter of course and further tried extensively in magnetic applications, etc. by taking advantage of superconductive properties which can generate a large electromagnetic force due to an interaction thereof with a magnetic field.
In addition to the above mentioned Y 123-type oxide superconductor, superior superconductive properties are successively confirmed in other RE 123-type oxides in which Y (yttrium) as a component thereof is replaced by various rare earth elements other than Y and are also widely studied to develop possible devices and instruments by applying these properties.
A "flux process", "melt-solidifying process" and the like are mainly used as a method to prepare RE 123-type oxide superconductors including the Y 123-type (hereinafter Y is included in the expression of rare earth elements or RE).
According to the "flux process" as a method for preparing oxide superconductors, the temperature of a relatively homogeneous supersaturated solution (molten liquid) of a starting composition (flux) comprising an oxide mixture of required components is gradually lowered to crystallize oxide superconductive crystals from the solution cooled below the solubility limit.
On the other hand, according to the "melt-solidifying process", a starting composition (flux) is heated beyond the peritectic temperature of an intended oxide to form a mixed state of solid and liquid phases, the temperature of which is then slowly lowered from the thus heated condition to cause a peritectic reaction so as to form oxide superconductive crystals.
In these processes, the initial constitution of the starting composition is adjusted to reside on a "123-211 (or 422) tie line" of a ternary phase diagram as will be described later (see FIG. 1) while keeping a sufficient but not excessive mass balance to yield a superconductive phase (123 phase) and to cause micro-dispersion of the 211 phase (in cases of Y, Sm, etc.) or the 422 phase (in cases of Nd, La, etc.), which functions as a magnetic flux pinning center in the superconductive phase to increase the critical current density (Jc), or keeping the mass balance to minimize a residue of unreacted Cu, Ba, etc.
It should be understood that the above mentioned "tie line" means a line on the phase diagram which shows a relationship between a crystalline phase composition and a liquid phase composition in the range where crystalline and liquid phases are equilibrated.
It is pointed out, however, that the thus prepared 123-type oxide superconductors, i.e., the RE 123-type other than Y, exhibit neither a sufficient critical temperature (Tc) nor a higher critical current density (Jc) in a magnetic field, although only the Y 123-type has relatively preferable superconductive properties.
As for the reason why, it is considered that the ionic radius of rare earth elements (RE), except Y, is relatively large and comparable to that of Ba, which causes the mutual replacement of RE and Ba when a molten raw material is cooled and solidified to form a superconductive phase, so that the chemical composition of the thus formed oxide crystals deviates from that of the 123 phase to be yielded.
FIG. 1 shows a "ternary phase diagram of 1/2Re.sub.2 O.sub.3 --BaO--CuO" in an atmosphere of oxides including RE, such as La, Nd, Sm, Pm, Eu, Gd, etc., having a relatively larger ionic radius. "A solid solution region of certain width" exists along a line extending to the upper right from the RE 123 phase, as shown in FIG. 1. The solid solution region exists to stabilize a phase of RE1+.sub.x Ba.sub.2-x Cu.sub.3 O.sub.y (x&gt;0, 6.0&lt;y&lt;7.2) which deviates from the RE 123 phase in the atmosphere. As is clear from the existence of the solid solution region, the mutual replacement of RE and Ba occurs in oxides of RE having a relatively larger ionic radius when the superconductive phase is solidified in an air atmosphere.
As shown in FIG. 2, the superconductive properties of the RE 123-type oxide superconductors change depending on the quantity of replacement (x) between RE and Ba.
Accordingly, the critical temperature thereof is lowered considerably as the quantity of replacement x increases and, on the other hand, is raised as the quantity of replacement x decreases, i.e., as the composition comes close to that of the RE 123-type oxide.
As is described above, it is considered that the mutual replacement of RE and Ba at a stage to form the superconductive phase by cooling and solidifying the starting material from a molten state thereof (a nucleating and growing stage) is a major cause for hindering the attainment of a higher critical temperature.
Then, there has been proposed an "Oxygen Controlled Melt Growth" process (hereinafter referred to as OCMG process) in which a starting composition is molten and solidified to promote crystal growth not in the ambient atmosphere but in an atmosphere where the content of oxygen in the atmosphere is controlled to a lower level.
It is known that RE 123 type oxide superconductors of Sm, Nd, etc. prepared by the OCMG process exhibit a higher critical temperature (Tc) and a higher critical current density (Jc) in a high magnetic field compared with the Y 123-type oxide superconductor of similar crystal structure because of a phenomenon in which the replacement of RE to a Ba site is controlled in an atmosphere having a low oxygen partial pressure.
In the OCMG process, however, it is quite disadvantageous from a standpoint of production that crystal growth should be performed in a controlled atmosphere of lower oxygen, although RE 123-type oxide superconductors having a higher critical temperature and higher critical current density in a magnetic field can be obtained because RE 123 crystals containing a lower quantity of replaced Ba are easily formed.
On the other hand, in the above described other process in which the starting composition is molten, cooled and solidified to form 123-type oxide superconductive crystals, the initial constitution thereof is adjusted to richen the 211 (or 422)-phase on the 123-211 (or 422) tie line so that grains of the 211 phase (or grains of the 422 phase) are dispersed in the 123 phase, thereby functioning as a magnetic flux pinning center to improve the critical current density (Jc).
The magnetic flux pinning center should be as fine as possible.
In this case, a trace amount of Pt or CeO.sub.2 is generally added to the starting composition to micronize grains of the 211 phase (or grains of the 422 phase) formed in the 123 superconductive phase.
However, even when the dispersed phase is micronized by a trace addition of Pt or CeO.sub.2, grains of the 211 phase (or grains of 422 phase) formed in the RE 123 phase are micronized at most to an extent of micron order in the RE 123-type oxide superconductors, except Y, which causes a problem, although grains of the 211 phase formed in the Y 123 phase are micronized to an extent of submicron order to exhibit preferable critical current density in the Y 123-type oxide superconductor.
After all, in order to impart "higher critical current density in a lower magnetic field" of a level of the Y 123-type oxide superconductor to the RE 123-type oxide superconductors, except Y, it is considered that grains of the RE 211 phase (or grains of the 422 phase) formed and dispersed in the RE 123 phase should be further micronized.